chapter-1-structure-and-bonding

ChapterIStructure&Bonding1-1ElectronConfigurationsofAtoms1-2ChemicalBonding&Valence1-3ChargeDistributioninMolecules1-4TheShapeofMolecules1-5Isomers1-6Resonance1-71-8AtomicandMolecularOrbitalsThestudyoforganicchemistrymustatsomepointextendtothemolecularlevel,forthephysicalandchemicalpropertiesofasubstanceareultimatelyexplainedintermsofthestructureandbondingofmolecules.Thismodule模块introducessomebasicfactsandprinciplesthatareneededforadiscussionoforganicmolecules.1-1ElectronicConfigurationsElectronConfigurationsinthePeriodicTableTheperiodictableshownhereisseverelytruncated.Thereare,ofcourse,overeightyotherelements.Thehalogensareoneelectronshortofavalenceshelloctet,andareamongthemostreactiveoftheelements.Intheirchemicalreactionshalogenatomsachieveavalenceshelloctetbycapturingorborrowingtheeighthelectronfromanotheratomormolecule.Thealkalimetalsarealsoexceptionallyreactive,butfortheoppositereason.Theseatomshaveonlyoneelectroninthevalenceshell,andonlosingthiselectronarriveatthelowershellvalenceoctet.Asaconsequenceofthiselectronloss,theseelementsarecommonlyencounteredascations(positivelychargedatoms).1-2-1IonicBondingWhensodiumisburnedinachlorineatmosphere,itproducessodiumchloride.Thishasahighmeltingpoint(800ºC)anddissolvesinwatertogiveaconductingsolution.Sodiumchlorideisanioniccompound,inwhichanelectronofsodiumatomwastransferredtoachlorineatomandgeneratesasodiumcationandachlorideanion.Electrostaticattractionresultsintheseoppositelychargedionspackingtogetherinalattice.Theattractiveforcesholdingtheionsinplacecanbereferredtoasionicbonds.1-2ChemicalBondingandValence1-2-2CovalentBondingWaterisaliquidatroomtemperature;carbondioxideandcarbontetrafluoridearegases.Noneofthesecompoundsiscomposedofions.Adifferentattractiveinteractionbetweenatoms,calledcovalentbonding,isinvolvedhere.Covalentbondingoccursbysharingofvalenceelectrons,ratherthananoutrightelectrontransfer.Similaritiesinphysicalproperties(theyareallgases)suggestthatthediatomicelementsH2,N2,O2,F2&Cl2alsohavecovalentbonds.Carbondioxideisnotablebecauseitisacaseinwhichtwopairsofelectronsaresharedbythesametwoatoms.Thisisanexampleofadoublecovalentbond.1-2-3ValenceThenumberofelectronsanatomgainorlosetoachieveavalenceoctetiscalledvalence.Thevalenceshererepresentthemostcommonform普通形式inorganiccompounds.Manyelements,suchaschlorine,bromineandiodine,areknowntoexistinseveralvalencestatesindifferentinorganiccompounds.1-3ChargeDistributionIftheelectronpairsincovalentbondsweresharedabsolutelyevenlytherewouldbenofixedlocalchargeswithinamolecule.AlthoughthisistruefordiatomicelementssuchasH2,N2andO2,mostcovalentcompoundsshowsomedegreeoflocalchargeseparation,resultinginbondand/ormoleculardipoles.1-3-1electronegativityDifferentatomshavedifferentaffinitiesfornearbyelectrons.Theabilityofanelementtoattractorholdontoelectronsiscalledelectronegativity.Fluorinehasthegreatestelectronegativityofalltheelements,andtheheavieralkalimetalssuchaspotassium,rubidiumandcesiumhavethelowestelectronegativities.Itshouldbenotedthatcarbonisaboutinthemiddleoftheelectronegativityrange,andisslightlymoreelectronegativethanhydrogen.Whentwodifferentatomsarebondedcovalently,thesharedelectronsareattractedtothemoreelectronegativeatomofthebond,resultinginashiftofelectrondensitytowardthemoreelectronegativeatom.Suchacovalentbondispolar,andwillhaveadipole.Thedegreeofpolarityandthemagnitudeofthebonddipolewillbeproportionaltothedifferenceinelectronegativityofthebondedatoms.ThusaO–HbondismorepolarthanaC–Hbond,withthehydrogenatomoftheformerbeingmorepositivethanthehydrogenbondedtocarbon.Likewise,C–ClandC–Libondsarebothpolar,butthecarbonendispositiveintheformerandnegativeinthelatter.Thedipolarnatureofthesebondsisoftenindicatedbyapartialchargenotation(δ+/–)orbyanarrowpointingtothenegativeendofthebond.1-3-2PolarCovalentBondsTheshiftofelectrondensityinacovalentbondtowardthemoreelectronegativeatomorgroupcanbeobservedinseveralways.Forbondstohydrogen,acidityisonecriterion.Ifthebondingelectronpairmovesawayfromthehydrogennucleustheprotonwillbemoreeasilytransferedtoabase(itwillbemoreacidic).Methaneisalmostnon-acidic,sincetheC–Hbondisnearlynon-polar.TheO–Hbondofwaterispolar,anditisatleast25powersoftenmoreacidicthanmethane.H–Fisover12powersoftenmoreacidicthanwaterasaconsequenceofthegreaterelectronegativitydifferenceinitsatoms.Electronegativitydifferencesmaybetransmittedthroughconnectingcovalentbondsbyaninductiveeffect.Thisinductivetransferofpolaritytapersoffasthenumberoftransmittingbondsincreases,andthepresenceofmorethanonehighlyelectronegativeatomhasacumulativeeffect.Forexample,trifluoroethanol,CF3CH2–O–Hisabouttenthousandtimesmoreacidicthanethanol,CH3CH2–O–H.1-3-2FunctionalGroupsFunctionalgroupsareatomsorsmallgroupsofatoms(twotofour)thatexhibitacharacteristicreactivitywhentreatedwithcertainreagents.Aparticularfunctionalgroupwillalmostalwaysdisplayitscharacteristicchemicalbehaviorwhenitispresentinacompound.Becauseoftheirimportanceinunderstandingorganicchemistry,functionalgroupshavecharacteristicnamesthatoftencarryoverinthenamingofindivid